This invention relates to an IC device with a built-in circuit for protecting internal information. More particularly, the invention relates to IC cards, logic ICs having a program-loaded memory, FPGAs (field programmable gate arrays) and other IC devices with a built-in security circuit for protecting internal information against theft and other unauthorized access to an opened IC.
Various technologies are known in the art of protecting internal IC information against theft. In order to prevent a third party from attempting unauthorized decoding of the contents of memories, various methods have been proposed, particularly in the field of IC cards. One of such methods is by checking for the coincidence between special codes so that unauthorized data accessing turns a failure. Another method is by encrypting the internal data. However, these approaches that rely upon certain data processing are totally ineffective if someone unpacks an IC to have the chip become exposed and scan the bare chip with an electron beam from an electron beam tester. As the result, the internal logic of the IC can be analyzed and subsequent theft of the internal information is also possible.
In IC cards, the use of a plurality of chips such as a CPU and a memory is common. In recent models of IC cards, a plurality of chips are integrated into a single system IC. In this class of circuit configurations, a memory IC is connected to a control IC (e.g. CPU) via a bus and the information in the memory can be analyzed by making a signal transfer between chips or detecting the signal on the bus.
In order to prevent the theft of design information by electron beam analysis of internal IC logic or by analysis of a program or data with reference to the signal transfer between IC chips or the signal on the bus, it has been proposed in Examined Japanese Patent Publication (kokoku) No. 68727/1993 that a coil be provided to distribute electromagnetic energy around the internal electronic circuit. If someone opens an IC by removing its case, the coil detects the event and the detection signal causes the internal data to be destroyed so that it is no longer accessible.
According to the approach proposed in Unexamined Published Japanese Patent Application (kokai) No. 200414/1995, a power line is allowed to run over the entire surface of the substrate so that if someone attempts to remove the IC chip from the substrate, the power line breaks, thereby erasing the information in an SRAM and other memories.
The Applicant for patent of the present invention previously proposed an invention entitled "IC card and IC card module" in Japanese Patent Application No. 128304/1997 (not laid-open), which was characterized by the provision of a circuit which, when an IC card encasing not only an IC chip but also a sensor such as a photodiode that detects the opening of the IC card is opened, is either disabled from performing all or some of its functions normally or abled for prohibiting access to the data in a memory.
However, none of the above-described methods of protecting the internal information of ICs by equipping them with a certain circuit or mechanism for protection against their unauthorized opening are effective against reverse engineering. Someone purchases several ICs of the same model, study and analyze some of them to identify the method of protection they use. Once this is accomplished, the other ICs are tampered with such that the protection circuit or mechanism will not function properly to provide security against the theft of the internal information.